Description: Forty-nine hydrothermal sulfide-sulfate rock samples from the Endeavour Segment of the Juan de Fuca Ridge, northeastern Pacific Ocean, were dated by measuring the decay of 226Ra (half-life of 1600 years) in hydrothermal barite to provide a history of hydrothermal venting at the site over the past 6000 years. This dating method is effective for samples ranging in age from ∼200 to 20,000 years old and effectively bridges an age gap between shorter- and longer-lived U-series dating techniques for hydrothermal deposits. Results show that hydrothermal venting at the active High Rise, Sasquatch, and Main Endeavour fields began at least 850, 1450, and 2300 years ago, respectively. Barite ages of other inactive deposits on the axial valley floor are between ∼1200 and ∼2200 years old, indicating past widespread hydrothermal venting outside of the currently active vent fields. Samples from the half-graben on the eastern slope of the axial valley range in age from ∼1700 to ∼2925 years, and a single sample from outside the axial valley, near the westernmost valley fault scarp is ∼5850 ± 205 years old. The spatial relationship between hydrothermal venting and normal faulting suggests a temporal relationship, with progressive younging of sulfide deposits from the edges of the axial valley toward the center of the rift. These relationships are consistent with the inward migration of normal faulting toward the center of the valley over time and a minimum age of onset of hydrothermal activity in this region of 5850 years.

Description: Massive portions of a ~ 300 °C black smoker (Finn) and a diffusely venting, intermediate temperature (~ 200 °C) chimney (Roane) recovered from the Mothra Hydrothermal Field on the Juan de Fuca Ridge have been analyzed in detail for bulk chemistry. The size of the recovered pieces of chimney enabled physical segregation and analysis of distinct chemical zonation within the structures. In particular, large variations in the concentrations of minor and trace elements in the different mineralogical zones provide new insight into the processes involved in chimney growth that are not evident from the bulk mineralogy. Systematic zonation of trace and minor elements is strongly influenced by redox conditions, multiple host phases, single or dual fluid source, and the mixing–cooling history of the fluids, even within an individual mineralogical zone. Distinct chemical microenvironments within the chimneys are recognized that provide a basis for detailed reconstruction of the changing conditions in the interiors of the chimneys during their growth. In Finn, 14 discrete chemical microenvironments are recognized in 80 of 90 samples based on high resolution zoning of major and trace elements. Four of the microenvironments are Cu-rich, three are Zn-rich, one is Fe-rich with low Cu, two are Ca-rich, three are SiO2-rich, and one is Ba and SiO2-rich. In Roane, 24 discrete chemical microenvironments are recognized. Six of the microenvironments are SiO2-poor (〈 1 wt.%) with low SO4, two have low SiO2 (1–10 wt.%) and low SO4, four have low SiO2 and are SO4-free, four have moderate SiO2, three are SiO2-rich, four have low to moderate Ba, and one is Ba-rich. The trace elements Se, Co, In, and Te do not display their typical simple associations with high concentrations of Cu. Variable Se concentrations in the most Cu-rich samples likely reflect variable Se/S ratios of the hydrothermal fluids in response to changing pH, oxidation state or mixing. Co and Te are strongly partitioned into pyrite instead of chalcopyrite indicated by concentrations of these elements remaining high even as the Cu content decreases. Indium also may be present either in chalcopyrite or in Zn sulfides ± chalcopyrite disease. In Roane, an unexpected correlation between Zn, Cd, Cu and Se is thought to reflect buffering of the fluids to a high pH by ammonia, which increases the saturation temperature of Zn sulfides and caused codeposition of Cu and Zn, with Cd enriched in Zn sulfides and Se enriched in Cu sulfides. The notable Ag enrichment in the Cu-rich samples from Finn are distinct from East Pacific Rise black smokers, possibly due to the lower temperatures (~ 300 °C vs. ≥ 350 °C) and higher fluid pH. Lower temperature assemblages in both Finn and Roane are enriched in Sb, Tl, Hg, Pb, ± Au, and ± Ag. Arsenic is enriched in some low-temperature zones but is most strongly correlated with Mo in pyrite; both elements are also absorbed from seawater as oxyanions. Uranium and V are also interpreted to have a seawater source and both may be present in clay minerals, together with the alkalies (Na, K, Rb, and Cs) while U may also be associated with sulfate minerals. Variable enrichments of Al2O3, U, and V in the silica-rich microenvironments within Finn and Roane indicate either seawater–hydrothermal fluid mixing or conductive cooling of hydrothermal fluid as the main process precipitating the silica. These findings have implications for the range of possible microbial habitats that may develop within chimney structures during their growth, which may include highly variable redox gradients and the availability of different nutrients.

Description: Centimeter‐scale mineralogical and chemical analyses of a diffusely venting, ∼280°C sulfide structure (called Roane) from the Mothra Hydrothermal Field on the Juan de Fuca Ridge reveal a complex growth history. These analyses document four well‐defined zones, which from the exterior to the interior of the structure include a barite‐silica zone (Fossil Worm Tube Zone), a silica‐sulfide zone (Silica Zone), a sulfide‐silica zone (Outer Sulfide Zone), and a sulfide ± gordaite ± silica zone (Inner Sulfide Zone). These features are a product of a myriad of processes that include extensive mineral replacement, ingress of seawater, fracturing and breakout of hot hydrothermal fluids, significant conductive cooling, and the sustained, broad‐scale outflow of warm fluids from the walls through a porous and permeable matrix. Roane lacks an open, throughgoing, chalcopyrite‐lined, central conduit. Instead, it hosts an anastomosing, discontinuous network of tortuous channels within the interior of a sponge‐like matrix of amorphous silica, sulfide, sulfate, and clay minerals. Megafaunal communities at the summit and sides of this mature, diffusely venting chimney provide a constructional framework for new growth. Isolation of hydrothermal fluid from seawater during progressive fossilization of megafauna and the early formation of barite‐silica assemblages promote flow of higher‐temperature fluids within the interior. Continued isolation of interior higher‐temperature fluids, through increased mineral precipitation and expansion of the structure, leads to a Zn sulfide + pyrite‐dominated permeable matrix and a network of tortuous channels that form a central porous conduit. This conduit is shielded by a weakly silicified, metal sulfide inner wall and a strongly silicified outer wall. The most outer portions of the walls are highly porous and sites of significant advection and mixing between hydrothermal fluids and seawater. The mineralogy and chemistry of Roane are strongly affected by ammonia‐ammonium buffering of the pH during cooling of the hydrothermal fluids. The two major results of the buffering are (1) precipitation of Zn sulfide at higher temperatures than are typical for bare‐rock systems, leading to the correlation of Cu, Zn, Cd, and Se and (2) increased deposition of amorphous silica and clay minerals.